The causes of "Gulf war syndrome" are still not known and probably never will be, experts believe.

Some 6,000 veterans have suffered unexplained poor health since the 1991 war, including depression and tumours.

But weeks before an inquiry is due to report and after a leak of a US probe said chemicals were to blame, UK experts said the cause was a mystery.

And Simon Wessely, director of the Gulf War Research Unit, claimed scientists might never understand the problem.

He said: "It is 14 years since the war and we have learnt a fair amount since then.

He added: "There are huge areas that remain unclear and I am afraid I suspect they will always remain unclear."

Illnesses

Prof Wessely, who is also professor of epidemiological and liaison psychiatry at the Institute of Psychiatry in London, said he could not comment on reports that a US inquiry had found the syndrome did exist and was caused by toxic chemicals.

Prof Wessely refuses to use the term syndrome but accepts the veterans have experienced a higher number of illnesses.

He said: "I am completely certain that there is no single cause."

'GULF WAR SYNDROME' - COMPETING THEORIES

Vaccines - Troops were given multiple vaccines before going to the Gulf, which some say could have overloaded the body.

Depleted uranium - DU is radioactive and was used in weapons. It can also lead to chemical poisoning.

Chemicals - Veterans could have come into contact with nerve agents from chemical weapons storage facilities and local pesticides.

Psychiatric disorders - Fighting wars can lead to depression and post-traumatic stress.

The Ministry of Defence, which does not recognise the syndrome as a medical condition, saying the symptoms are too varied to be considered part of a wider syndrome, has also refused to comment on the US findings.

Controversy has surrounded so-called Gulf war syndrome since veterans began to experience more ill health than military personnel who had served in previous and subsequent conflicts.

While veterans have not experienced more heart disease or cancer, the levels of general ill health, including mood swings, memory loss, lack of concentration and night sweats, have been 20% higher.

Some have blamed the high number of vaccines and medication given to the armed forces to protect them against a variety of illnesses, including anthrax.

Others have suggested it was caused by chemicals, such as pesticides and nerve agents, or exposure to depleted uranium which was used in weapons. But UK researchers have dismissed the theories.

Vaccines

Professor Brian Spratt, chairman of the Royal Society working group on depleted uranium munitions, said the exposure would have been "too low".

And Professor Mark Peakman, from Guy's, King's and St Thomas's School of Medicine, who has done research on the effect of vaccines, said he did not believe the multiple vaccines administered were to blame either.

Personnel received vaccines against biological weapons threats

However, he admitted the theory that vaccines and chemicals interacted in some way "still lurks".

Shaun Rusling, vice chairman of the National Gulf Veterans and Families Association, accused the scientists of having "cold hearts" and "closed minds".

"The US has been far more advanced at looking in to this. The clinical scientific evidence is irrefutable.

"The US study said it wasn't caused by the stress of fighting the war but the chemicals we came into contact with.

"It is disgusting the British government and scientists don't admit this."

The Gulf war syndrome inquiry, funded by anonymous donors and headed by former judge Lord Lloyd of Berwick, is due to unveil its findings in the next few weeks.